Electrolytes for chemical silvering comprising silver nitrate and thiourea or similar compounds are used as electrolytes for metal deposition. In the case of silver electrolytes especially, however, alternatives to silver nitrate or silver cyanide are wanted.
Electrolytes for chemical silvering comprising silver nitrate and thiourea or similar compounds have a tendency to explosive decomposition on drying.
Virtually only alkaline cyanide-containing electrolytes comprising silver cyanide are used for electrodeposition. These electrolytes have the disadvantage of their high toxicity. In addition, masking lacquers of electronic components are attacked by these electrolytes. Furthermore, these electrolytes permit only relatively low current densities/deposition rates. Nitric acid electrolytes comprising silver nitrate are unsuitable for the electrodeposition of metals, since the nitrate is destroyed by cathodic reduction.
Other silver salts obtainable on an industrial scale, such as silver chloride or silver sulfate, are unsuitable for preparing chloride or sulfated electrolytes, since the water solubility of these silver compounds is too low.
In contrast thereto, the high water solubility of silver methane sulfonate is known, so that this compound and specific silver alkylsulfonates or arylsulfonates are of interest as an alternative to the known electrolytes as an electrolyte for the electrodeposition of silver.
WO 85/03530 describes a process for preparing heavy metal carboxylates. Page 6, line 14 also mentions silver as one of the heavy metals. Page 14, line 27 to page 15, line 5 further describes the fact that sulfonic acid salts, e.g. of p-toluenesulfonic acid, can also be prepared by this process. However, none of the exemplary embodiments indicates the use of silver.
In the paragraph extending over pages 3 and 4, it is stated that the electrolyte shall comprise an ionic component which readily dissociates into anions and cations, in particular alkali metal halides, in particular sodium chloride, being proposed.
As is known, silver halides are sparingly soluble in aqueous solutions, so that in the event of anodic dissolution of silver in a sodium-chloride-containing electrolyte, silver chloride would immediately precipitate.
U.S. Pat. No. 5,618,404 describes an electrolytic process for preparing lead and tin sulfonates which, however, includes a complex separation of the anodic region from the cathodic region by cation and anion exchange membranes.
JP 0 521 38 54 A2 (CA 119:270800) discloses a process for preparing silver sulfonate salts. A solution of silver carbonate in methyl cyanide is reacted with a solution of methanesulfonic acid in methyl cyanide, in order to obtain 93% pure silver methylsulfonate. Furthermore, it is also known to prepare silver methanesulfonate from aqueous methanesulfonic acid and silver carbonate or silver oxide, or from aqueous methanesulfonic acid, silver powder and, if appropriate, hydrogen peroxide (U.S. Pat. No. 5,491,247 A). However, the above mentioned processes are very costly, since the silver is generally initially dissolved in nitric acid in order to prepare the silver carbonate or silver oxide and then to react this compound with methanesulfonic acid. Although the preparation from the metal using hydrogen peroxide is cheaper, a large metal surface area is necessary for acceptable reaction times. This determines the necessity of using expensive metal powder. Furthermore, there is a serious disadvantage in that greater or lesser amounts of methanesulfonic acid are decomposed to form sulfuric acid and carbon dioxide, as a result of which relatively costly hydrogen peroxide is consumed.